<tex> regula sigma 14: \zeta(s) = \sum^{\infty}_{1}{n^s} </tex>
<tex> regula sigma 15: \binom{n}{k} = \frac{n!}{(n-k)! \cdot k!}</tex>
<tex> regula sigma 16: f(\sum^{n}_{1}{p_i x_i}) \leq \sum^{n}_{1}{p_i \cdot f(x_i)}</tex>

<tex> regula sigma 17: f_n = \frac{(\frac{1 + \sqrt{5}}{2}) ^ n - (\frac{1 - \sqrt{5}}{2}) ^ n}{\sqrt{5}}</tex>
<tex> regula sigma 18: P(|X - \mu| \geq k \sigma) \leq \frac{1}{k^2}</tex>
<tex> regula sigma 19: \sum_{cyc}{x^t(x-y)(x-z) \geq 0}</tex>
<tex> regula sigma 20: a ^ {p - 2} \equiv \frac{1}{a} (\text{mod }p)</tex>
<tex> regula sigma 21: (n - 1)! \equiv -1(\text{mod }n)</tex>